In an intrusion alarm system for the protection of a volumetric space such as a room or other walled enclosure, transmitting and receiving transducers are often mounted in the ceiling of a room or other enclosure for propagation of an energy pattern into the protected space. Strong reflections from the general area of the floor directly below the transmitting transducer tend to be reflected back into the receiving transducer, causing high levels of received energy which, if modulated by vibrations or moving air, can cause high system noise levels. In addition, a transducer having a normal small radiating surface (nominally one wavelength) directed down from the ceiling produces a pattern on the floor which uniformly decreases in intensity for increasing angles outwardly of the boresight axis. Since distance from the transducer to the floor also increases for increasing angles, intruder detection sensitivity decreases correspondingly. In order to obtain uniform detection sensitivity, it would be desirable to compensate for the increased range by providing a pattern which increases for increasing angles outwardly of the boresight axis.